Abstract We present a systematic study of the effect of metallicity
on the stellar spectral energy distribution (SED) of O main sequence
(dwarf) stars, focussing on the hydrogen and helium ionizing continua,
and on the optical and near-IR lines used for spectral classification.
The spectra are based on non-LTE line blanketed atmosphere models with
stellar winds calculated using the CMFGEN code of
Hillier & Miller (1998). We draw the following conclusions. First, we find
that the total number of Lyman photons emitted is almost independent
of line blanketing effects and metallicity for a given effective
temperature. This is because the flux that is blocked by the forest of
metal lines at
Å is redistributed mainly within the
Lyman continuum. Second, the spectral type, as defined by the ratio of
the equivalent widths of
4471 and
4542,
is shown to depend noticeably on the microturbulent velocity in the
atmosphere, on metallicity and, within the luminosity class of
dwarfs, on gravity. Third, we confirm the decrease in
for a
given spectral type due to the inclusion of line blanketing recently
found by e.g. Martins et al. (2002). Finally, we find that the SED below ~
450 Å is highly dependent on metallicity. This is reflected
in the behaviour of nebular fine-structure line ratios such as [
]/[
] 15.5/12.8 and [
]/[
] 9.0/7.0
. This dependence
complicates the use of these nebular ratios as diagnostic tools for
the effective temperature determination of the ionizing stars in
regions and for age dating of starburst regions in galaxies.